This section is intended to provide a background to the various embodiments of the technology described in this disclosure. The description in this section may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and/or claims of this disclosure and is not admitted to be prior art by the mere inclusion in this section.
In Time Division Duplex (TDD) wireless systems, such as Time Division-Synchronous Code Division Multiple Access (TD-SCDMA) systems, Time Division-Long-term Evolution (TD-LTE) systems and TD-LTE Advanced (TD-LTE A) systems, the uplink/downlink channel reciprocity is exploited to estimate the downlink channel based on knowledge of the uplink channel.
However, in practical implementation, the uplink/downlink channel reciprocity may be not ideal due to a signal response characteristic difference between transmission and reception paths inside a transceiver. To ensure the reciprocity between uplink and downlink channels, it is necessary to determine and compensate for the signal response characteristic difference between the transmission and reception paths inside the transceiver.
In addition, within a multi-antenna transceiver comprising multiple pairs of transmission and reception paths corresponding to its multiple antennas, different transmission/reception paths, which consist of different Intermediate Frequency (IF) chains, different Radio Frequency (RF) chains and possibly different feeder cables, may exhibit different amplitude attenuation and phase shift characteristics. This in turn results in different reciprocity characteristics for different working path loops (that is, different pairs of transmission and reception paths have their respective signal response characteristic differences between transmission and reception paths). Accordingly, inner-transceiver antenna calibration has been introduced for the multi-antenna transceiver to compensate for the reciprocity characteristic inconsistency among the transceiver's different working path loops.
To improve cell throughput and cell-edge throughput effectively, the so-called Coordinated Multiple-Point (CoMP) transmission technology, which allows for mitigated co-channel interference and/or increased diversity gain by coordinating transmissions from more than one transceivers (for example, transceivers corresponding to sectors of the same RBS, or transceivers corresponding to sectors of different RBSs), has been introduced to LTE systems for downlink communications.
As a typical scheme of the CoMP transmission technology, the Coherent Joint Transmission (C-JT) scheme as illustrated in FIG. 1 enables a User Equipment (UE) to conduct a co-phase combination of transmitted signals from multiple transceivers by having the multiple transceivers perform joint pre-coding and beam-forming. As the C-JT scheme involves at least two antennas within different transceivers, there is a need to compensate for reciprocity characteristic inconsistency among different working path loops within different transceivers. To do this, inter-transceiver antenna calibration shall be applied.
For the inter-transceiver antenna calibration, WO20110544144A1 and CN102149123A propose a solution called node-assistant inter-transceiver antenna calibration, wherein a wireless-enabled assistant node (such as a relay, a micro station or a UE) is introduced for inter-transceiver antenna calibration as illustrated in FIG. 2. More specifically, an inter-transceiver antenna calibration process is divided into an inter-transceiver antenna reception calibration process, which compensates for signal response characteristic differences among reference reception paths of different transceivers, and an inter-transceiver antenna transmission calibration process, which compensates for signal response characteristic differences among reference transmission paths of different transceivers. In the case of inter-transceiver antenna reception calibration, transceivers involved in CoMP transmission receive a calibration signal from the assistant node and calibrate their reference reception paths according to a comparison between the calibration signal and its distorted versions received at the transceivers. In the case of inter-transceiver antenna transmission calibration, the assistant node receives orthogonal calibration signals from the transceivers, determines calibration parameters for calibrating the reference transmission paths of the transceivers according to a comparison between the orthogonal calibration signals and their distorted versions received by the assistant node, and feeds the calibration parameters respectively back to the transceivers.
Thus, without the assistant node, it is impossible to implement the node-assistant inter-transceiver antenna calibration among the transceivers involved in CoMP transmission. Moreover, as propagation paths between respective transceivers and the assistant node differ from each other, the distortions of the calibration signal by the propagation paths cannot be cancelled out for inter-transceiver antenna calibration, which is intended to compensate signal response characteristic differences among reference transmission/reception paths of different transceivers. As a result, the node-assistant inter-transceiver antenna calibration solution, which is based on the comparison between the calibration signal and its distorted versions received over the different propagation paths, inherently suffers from calibration inaccuracy.
In addition, to carry out the inter-transceiver antenna calibration, the node-assistant inter-transceiver antenna calibration solution requires one assistant node to be deployed for each CoMP set. For a typical radio network which may comprise hundreds or thousands of CoMP sets, efforts and costs incurred by deploying assistant nodes are considerable.
For the inter-transceiver antenna calibration, CN102843173A discloses another solution called transceiver-assistant inter-transceiver antenna calibration as illustrated in FIG. 3, wherein one of the transceivers involved in CoMP transmission is selected to work as the assistant node in the node-assistant inter-transceiver antenna calibration solution. However, as the underlining principle of the transceiver-assistant inter-transceiver antenna calibration is the same as that of the node-assistant inter-transceiver antenna calibration, the distortions of the calibration signal by propagation paths between the selected transceiver and respective transceivers other than the selected one cannot be cancelled out, either. Consequently, the calibration inaccuracy still exists. In addition, the transceiver-assistant inter-transceiver antenna calibration cannot be implemented in a scenario where it is impossible to find a third transceiver that can work as an assistant node for only two transceivers involved in CoMP transmission.